One example of a polish apparatus for polishing objects such as a semiconductor wafer is a CMP (chemical mechanical polishing) apparatus. Polishing is carried out by moving the semiconductor wafer held by a polish head over a polish cloth. The polish head is provided with an annular retainer ring on its outer peripheral portion for holding the semiconductor wafer.
The polish head typically controls the polish profile by applying a constant pressure on the semiconductor wafer while applying controlled pressure on the retainer ring as well during the polishing process. When high pressure is applied to the retainer ring, the wear of the retainer ring becomes uneven and typically results in an increased clearance between the semiconductor wafer and the retainer ring. As a result, the pressure applied to the retainer ring becomes less effective which makes it difficult to maintain the desired polish profile.
Thus, increasingly high pressure needs to be applied to the retainer ring in order to obtain a polish profile close to the desired profile. However, application of high pressure accelerates the wear of the retainer ring itself.
On the other hand, the increase in the clearance between the semiconductor wafer and the retainer ring can be inhibited by reducing the diametrical width of the retainer ring. However, increasingly high pressure needs to be applied to the retainer ring in order to obtain a polish profile close to the desired profile since the area of contact between the semiconductor wafer and the polish cloth is reduced. This significantly reduces the life of the retainer ring.